1. Field of the Invention
The present invention is related to a Force-Rebalance Coriolis Vibrating Gyroscope (CVG), and more particularly, to an axisymmetrical gyroscope such as a ring, a cylindrical, a hemispherical, or tuning forks gyroscopes.
2. Description of the Related Art
A gyroscope is a device for measuring or maintaining orientation. Applications of gyroscopes include inertial navigation (INS), stabilization of ground vehicles, aircraft, ships, and line of sights, piloting etc. . . .
Conventional vibrating gyroscopes are disclosed in the following U.S.
Patents: U.S. Pat. No. 6,698,271, U.S. Pat. No. 7,120,548, U.S. Pat. No. 7,240,533, U.S. Pat. No. 7,216,525, U.S. Pat. No. 7,281,425, U.S. Pat. No. 5,597,955. Vibrating gyroscopes (i.e., gyroscopes based on vibrating structures) have many advantages over traditional spinning gyroscopes (i.e., based on a rapidly spinning mass), but also over gyroscopes based on fiber-optic or laser technologies.
Vibrating gyroscopes have a stronger structure with lower power consumption than conventional gyroscopes. They are made of only few parts, more or less near 5, whereas conventional gyroscopes have more than 20 parts. Consequently, they are cheaper and can be mass produced. Some of them belong to the Micro Electro Mechanical Sensors (MEMS) technology, and size of less than 1 mm is currently achieved. Unfortunately, reduction in size does not mean improvement in performance. However, this MEMS gyros lead to integrated and cheaper systems as required for instance for automotive market and consumer electronics.
A variety of geometries of vibrating structures can be used for manufacturing sensitive elements of vibrating gyroscopes. For example, rings, tuning forks, beams, thin-walled cylinders and hemispheres are used in vibrating gyroscopes. These vibrating structures can be made out of different materials: metal, piezoelectric ceramics, fused quartz (fused silica), thin-film silicon layers, etc.
Top of the range performances are observed on gyroscopes based on high-Q resonators with strict axial symmetry. Typical shapes are ring, hemisphere and cylinder, and vibration modes used are typically the second order vibrations (often named Cos(2θ) and sin(2θ) vibrations). Achievements based on all those ring-like resonators are much easier when considering a second order vibration, but other vibration orders could be available.
In case of an axisymmetrical double tuning forks such as described in U.S. Pat. No. 5,597,955, vibration mode order is 1 instead of 2.
In any case, axis of sensitivity of these gyroscopes is their axis of symmetry (i.e., the whole resonator shape can be created by rotating a partial shape around a straight line named axis of symmetry). Typically, this axis is denoted as Z-axis.
In the particular case of ring-like vibrating gyroscopes and second order vibration mode, fluctuations occur in such a way that the resonator is stretched into an ellipse along its X-axis during the first oscillations half-cycle and along the axis perpendicular to the X-axis in the second oscillation half-cycle (see FIG. 1A). In the course of these oscillations a standing wave vibrates inside the ring. The standing wave is characterized by four anti-nodes and nodes, distributed in a circular profile of the resonator at equal positions separated by 45 degrees angles.
The nodes and the anti-nodes are located in an alternate order, so a node is located between two anti-nodes, and the anti-node is located between the two nodes. In case of power balancing waves (i.e., a force-rebalance mode), an elastic wave is excited at the second vibration mode of the resonator with a given amplitude and the elastic wave is stabilized by an AGC system (Automatic Gain Control).
In the particular case of an axisymmetrical double tuning fork and first order vibration, beams are stretched through a bending mode and physical angle between X-axis and Y-axis is 90 degrees.
Whatever the axisymmetrical gyroscope, ring-like or double tuning fork as previously mentioned, the rotation around the sensitive axis creates Coriolis' forces:{right arrow over (F)}c=2m[{right arrow over (Ω)}Λ{right arrow over (v)}]where {right arrow over (F)}c is a vector of the Coriolis force, m is a vibrating mass, {right arrow over (Ω)} is a velocity of rotation of the resonator around its axis of symmetry, and {right arrow over (v)} is a linear velocity vector of the structural elements in the process of vibration, the symbol “Λ” denotes vector product.
Effect of Coriolis forces is to create Coriolis measured mode of (i.e., sense mode) oscillations. Amplitude of these oscillations is proportional to the angular velocity in |{right arrow over (Ω)}| of rotation. The Coriolis mode is oriented along the Y-axis when the excited mode (i.e., drive mode) is oriented along the X-axis.
In case of ring-like resonators, Coriolis mode nodes are located at the anti-nodes of the excited mode, and Coriolis mode anti-nodes are located at the nodes of the excited mode. Spatially, these two modes are rotated by an angle of 45 degrees to each other (see FIG. 1B).
The simplest way to operate theses modes is named “the force-rebalance mode”. A description is given in IEEE 1431. The drive mode is controlled with a constant amplitude and the Coriolis mode is controlled through a closed loop which parameters are the quadrature signal and angular inertial velocity |{right arrow over (Ω)}| Source of the quadrature is the mismatch in the two modes frequencies. The mismatch in frequencies is important because this leads to drift errors and poor performances.
Because of symmetry, axisymmetrical resonators' mismatch in frequencies and damping are theoretically zero. However, all gyroscopes described in the above patents have some imperfections due to material inhomogeneities, due to machining imprecision, but also due to drive and sense elements misplacements.
All these errors are responsible of mismatch in frequencies, damping and mismatch in damping. Thus, a significant zero-bias error appears which depends on temperature and time.
Conventional methods of compensation for dependency of the zero-bias on temperature use calibration, as disclosed in U.S. Pat. No. 7,120,548 and U.S. Pat. No. 7,240,533.
This process of calibration is time consuming and expensive. Additionally, materials ages and their properties change over time. As a result, conventional calibrations are not robust and effective. Other methods of compensation for zero-bias use internal signals of wave control system to compensate for the initial offset (U.S. Pat. No. 6,698,271). However, this method compensates only one source of error and does not compensate for other sources, such as effect of mismatch in damping and frequencies.
Accordingly, there is a need in the related art for a cost-effective and precise method for compensation of a zero-bias in a gyroscope.